Wireless charging offers a convenient way to provide power to electronic devices. A wireless charger transmits a wireless charging signal that is an electromagnetic signal of a certain frequency, and a relatively high power, such as 10 W. Electronic devices can receive the wireless charging signal and extract power therefrom, and use the received power to operate the device and/or charge a battery associated with the device. These actions are referred to herein as “charging” the device. There is no requirement to connect a physical cable between the device and a wireless charger. Each device being charged has an antenna for receiving power transmitted using the wireless charger's antenna, or resonator.
Efficiency of power transfer in wireless charging is affected by several factors. When a device is being charged, its antenna is in proximity to the wireless charger's resonator. Efficiency of power transfer from the charger to the device is affected by how efficiently the charger and device antennas are coupled magnetically. Coupling efficiency depends on what amount of magnetic flux generated by the wireless charger is captured and used by the device being charged. In the ideal case, where all magnetic flux generated by the charger is captured by the device, the ideal efficiency of 100% power transfer would be achieved (not considering other losses, for example resistive losses within the charger or device). In reality, only small fraction of magnetic flux generated by the charger is actually used to excite a current in the device antenna. This is mainly because magnetic flux from the charger is scattered widely in the surrounding space and it is difficult to control its spatial distribution.
FIG. 1 shows an example of a wireless charger 10 including a charging surface 12. The charging surface is substantially planar and contains a resonator 14 that is also substantially planar. Other components of the wireless charger 10, such as transmitting circuitry, controlling circuitry and other components, are not shown. An antenna 16 of a device being charged is shown. When a current is passed through the resonator 14, a magnetic field is produced that is illustrated by magnetic field lines 18. In practice, an alternating current of a certain frequency is passed through the resonator 14 such that the magnitude and direction of the magnetic field changes over time. It can be seen that the antenna 16 captures only a small fraction of the magnetic flux, and a considerable part of the flux is dissipated in the surrounding space.